There seem to be three very similar expressions of surprise - おや, あれ, and あら. Is there much particular difference? So far I've gathered that あれ is almost always genuine surprise, whereas あら and おや can express more broad ranges of emotion, and that あら is a little more feminine.
The 3 have very similar meaning that even Japanese sometimes can't distinguish.
It just describe surprising.
It's almost the same as おや but feminine.
It's slightly different, it means surprising when a difference is found with something, like your memory. e.g. あれ? an egg should have been in refrigerator! who ate it!